Extension of the methodology for the derivation of equipment impairment factors from instrumental models for wideband speech codecs
|Publication Date:||1 April 2009|
It is assumed that the reader is familiar with [ITU-T P.834].
The extension of the methodology described in this Recommendation is intended to derive wideband equipment impairment factors Ie,wb quantifying the impairment introduced by wideband (50-7000 Hz) speech codecs. Such wideband equipment impairment factors have been introduced as a simplified measure of the degradation introduced by wideband speech codecs on the integral transmission quality from mouth to ear. They are in no way an exact description of the effects related to each individual codec or codec tandem, which may be very diverse in their perceptual nature. Instead, they represent the relative degradation in comparison to other impairments occurring in a connection.
In order to provide guidance on the quantitative amount of impairment introduced by such codecs, a framework of wideband equipment impairment factor values for several wideband speech codecs has been derived by ITU-T, see Appendix IV to [ITU-T G.113]. The derivation of these values was based on many subjective listening-only tests carried out in different test laboratories, so as to guarantee stable values for all the codecs under investigation so far, as well as their relations to each other. If new equipment impairment factor values for different codecs have to be derived, then the overall consistency with the established framework is of primary importance if results are to be obtained that are valid for network planning. The methodology described in this Recommendation was designed to fulfil this requirement.
So far, wideband speech codecs are the only wideband elements for which impairment factors have been derived. For asynchronous tandems of multiple codecs of the same type, or of multiple codecs of different types, it is assumed that individual equipment impairment factors are additive. The overall equipment impairment factor for the chain of codecs is then calculated as a simple sum of all the individual Ie,wbs. Experimental test data collected provide evidence that this simple additivity is not satisfied for all the potential combinations of codecs. These deviations from the pure additivity property are an item for further study. Impairment factor values for other wideband network elements, resulting in other types of degradations and to be used in a wideband version of the E-model, have not yet been defined and are for further study as well.
ITU-T currently recommends two methodologies for the derivation of wideband equipment impairment factors: this Recommendation and [ITU-T P.833.1]. The methodology described in [ITU-T P.833.1] is based on the results of auditory listening-only tests. The present approach, which is based on instrumental models (so-called "objective methods"), requires that the models used provide valid estimations of auditory judgements. Only in this case will the derived wideband equipment impairment factors be valid. The described methodology has been tested with the model defined in [ITU-T P.862.2], and results were found to be satisfactory in most cases.